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Journal of Applied Physics

Experimental study on stacked Josephson tunnel junction arrays under microwave irradiation

Alexander M. Klushin, H. Kohlstedt

Published Jan 1, 1995• 15 citations
Publisher

Abstract

We investigated the behavior of vertically stacked tunnel junctions in a circuit suitable for voltage standards. Such three-dimensional (3D) circuits might offer a considerable reduction of chip size when compared with conventional, two-dimensional circuits. Up to 84 series connected stacks with four junctions in a stack were incorporated into a microstripline. In the current-voltage characteristic, we measured quantized current steps until 100 mV upon applying microwave irradiation in the W band. We observed both chaotic and nonchaotic behavior. Whether chaos or nonchaotic behavior is observed under microwave radiation depends on the coupling strength which is defined by the intermediate niobium layer thickness between the junctions of a stack. Large thickness corresponds to weak coupling and absence of chaos. We showed that 3D circuits are promising for voltage standard application.

DOI
10.1063/1.359351

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We investigated the behavior of vertically stacked tunnel junctions in a circuit suitable for voltage standards. Such three-dimensional (3D) circuits might offer a considerable reduction of chip size when compared with conventional, two-dimensional circuits. Up to 84 series connected stacks with four junctions in a stack were incorporated into a microstripline. In the current-voltage characteristic, we measured quantized current steps until 100 mV upon applying microwave irradiation in the W band. We observed both chaotic and nonchaotic behavior. Whether chaos or nonchaotic behavior is observed under microwave radiation depends on the coupling strength which is defined by the intermediate niobium layer thickness between the junctions of a stack. Large thickness corresponds to weak coupling and absence of chaos. We showed that 3D circuits are promising for voltage standard application.
Stacked Josephson junction arrays located at the rf current antinodes of a superconducting microstripline resonator were investigated. The circuits were designed to obtain highly accurate voltages at cm-waveband microwave driving and low rf-power level. Nb-Al/AlO/sub x/-Nb Josephson junctions with tunneling area of 60 /spl mu/m/spl times/300 /spl mu/m and critical current density up to 7 A/cm/sup 2/ were used. We compared the average voltages of the rf induced maximal current steps in arrays with up to three junctions per stack. Quantized current steps up to 300 mV were observed under microwave irradiation at frequency of 19.20 GHz in circuits with 384 three-junction stacks. The dynamics of stacked tunnel junctions in the presence of rf-currents will be discussed with respect to application as voltage standards.
Absbuct- Vertically stacked Josephson tunnel junctions with a high number of junctions (up to n = 20) were realized using a fabrication process based on the NbIAl technology. An experimental study aimed toward their potential applications was made. First, we report on a series of dc measurements, with the main purpose of studying the quality of the I-V curve as a function of the number of barriers in the stack; we found that the junction quality slowly becomes worse as we increase this number. In fact, some thermal effects appear, which result in an increase of the subgap current and a decrease of the width of the current jump at the total gap voltage. Further, we found a low frequency voltage noise on devices with n > 10 that depends Roberto Monaco, Albino Polcari, and Lucia Capogna on the junction area and on the substrate nature. Later, we discuss some RF properties of stacked junctions irradiated with a microwave signal. These measurements show that they seem to behave independently. Once again, the thermal effects drastically influence the response to an external RF drive and seem to limit the use for applications of these devices.
Microwave-Induced Characteristics of (NbN/TiNx)N/NbN{({\rm NbN/TiN}_{\rm x})}_{N}/{\rm NbN}(NbN/TiNx​)N​/NbN StackedJosephson Junction Arrays
Stacked (NbN/TiNx)N/NbN (N = 2, 3, 4, 5 and 10) Josephson junctions and their arrays have been fabricated on a Si chip and their microwave-induced characteristics were measured at a temperature around 10 K. To achieve a vertical and smooth edge of an NbN - TiNxmultilayer, an inductively coupled plasma reactive ion etching (ICP-RIE) technique was used. Current-voltage characteristics measured for stacks of (NbN/TiNx)N/NbN Josephson junctions without microwave indicated that critical currents for those junctions distribute in a certain range. We defined Ic- spread as (IcMAX- IcMIN)/IcAverage to measure the distribution of Ic. The Ic- spread was minimized when the thickness of intermediate electrodes was d = 50 nm. We also evaluated values of Ic- spread for stacks prepared at different fabrication runs as a function of the number of junctions in a stack N. As a result, we found N = 3 is the maximum for practical use. For arrays, we obtained a constant-voltage step height of about 2 m A for 2 048 stacks of double-barrier junctions (N = 2), and about 1 mA for 1 024 stacks of triple-barrier junctions (N = 3).
Josephson voltage standards utilize microwave-induced constant voltage steps occurring due to the ac Josephson effect. Existing standards can be considerably simplified and their accuracy improved by using a large number of series-connected Josephson tunnel junctions which are operated in the zero current step mode. For this purpose superconducting millimeter wave integrated circuits have been designed, fabricated, and tested. The circuits consist of a broadband taper between the rectangular waveguide and the planar structure, the Josephson junction series, a well-matched load, and dc pads. Circuits with various numbers of junctions have been fabricated by photo-lithographic techniques and tested at 4.2 K in liquid helium. The version with 1474 junctions produced voltages up to 1.2 V when operated at 90 GHz.
On a slice of 200–300-nm-thick Bi2Sr2CaCu2O8+x, stacks of intrinsic Josephson junctions are patterned, connected to each other, and integrated with a bow-tie antenna and some chokes. Over an area of 150 μm by 170 μm, we have been able to fabricate up to 256 stacks with over 11 000 junctions. Such three-dimensional arrays of intrinsic Josephson junctions respond sensitively to irradiation at 760 GHz from a far-infrared laser, showing clear zero-crossing Shapiro steps up to 0.75 V. In addition, possible applications in quantum voltage standards are discussed here. The high density of integration of the Josephson junctions is also promising for many other applications.
The configuration of a robust, high-security system has become increasingly important because, as computer processing speeds continue to improve, the vulnerabilities of cryptographic systems that rely on conventional pseudo-random-numbers have raised concerns regarding potential leaks of confidential information. To establish such a secure system, high-quality and high-speed random numbers are essential. Here, high-quality random-numbers are defined as statistically unbiased, non-reproducible, and unpredictable values. One approach to generating such numbers is to exploit physical random phenomena, such as chaos. In particular, chaos in Josephson junctions (JJs) under radio-frequency (RF) irradiation has been identified as a suitable mechanism for random-number generation. In this study, we considered RF-biased intrinsic Josephson junctions (IJJs) in a mesa-type Bi2_{2}2​Sr2_{2}2​CaCu2_{2}2​O8+δ_{8+\delta }8+δ​single crystal. Since the interactions among stacked IJJs must be considered, it is insufficient to evaluate their properties using the conventional resistively and capacitively shunted junction (RCSJ) circuit model. Instead, we numerically investigated the chaotic voltage of an IJJ stack irradiated with higher frequencies than those typically used in conventional RF systems, employing the capacitively coupled JJ model with diffusion current, which incorporates interlayer interactions. Based on the time-series output voltage of the IJJ, we then generated random-numbers and evaluated their quality using statistical tests.